/Engine/Source/Runtime/Engine/Classes/Engine/StreamableRenderAsset.h
Register a set of callbacks to get notified when new mips/LODs start streaming in and when streaming is complete. If the target mips/LODs are already streamed in, CallbackStreamingDone is called immediately and CallbackStreamingStart is not called.
void RegisterMipLevelChangeCallback ( [UPrimitiveComponent](API\Runtime\Engine\Components\UPrimitiveComponent) * Component, float TimeoutStartSecs, FLODStreamingCallback && CallbackStreamingStart, float TimeoutDoneSecs, FLODStreamingCallback && CallbackStreamingDone )